Dental handpieces to cut the root canals of teeth have a configuration in which a cutting tool called a file is rotated by a motor.
Here, the file that cuts the root canals of teeth has a thin long form. When the file bites into a tooth at the time of cutting the root canal of the tooth, a force in a twist direction is applied to the file, resulting in a problem that the file is easily broken.
Thus, there has been proposed a method for preventing biting of the file into the root canal of a tooth and breakage thereof by repeating forward and backward rotations in which the file is rotated in one direction for a given time, and thereafter rotated in an opposite direction (simply referred to as “forward and backward rotations” below) (e.g., see Patent Literature 1). Repeating the forward rotation and the backward rotation as described above is sometimes called reciprocating rotation. The forward rotation and the backward rotation do not indicate rotation in a particular direction, but a preceding rotation direction is referred to as forward rotation.
In the method of performing the forward and backward rotations according to Patent Literature 1, the file is rotated clockwise or counterclockwise through a desired first rotation angle, and subsequently rotated in a direction opposite to the first rotation angle through a second rotation angle. The first rotation angle is larger than the second rotation angle such that cut debris removed from the root canal is ejected upwardly from the surface of the root canal when the file is advanced in cleaning the root canal.